Heat-shrinkable film and tubing

ABSTRACT

Heat-shrinkable films and heat-shrinkable tubing of linear polymeric esters that shrink at least 2 1/2 times as much along one of the longitudinal and transverse axes of the films and tubing as they do along the other. The films and tubing shrink between 2 and 20 percent in the direction of least shrinkage and at least 20 percent in the direction of most shrinkage. A film of the invention is prepared by stretching it in a first direction at a temperature that is above the second order transition temperature for the polymer of the film, then thermally conditioning the film while it is in the stretched condition by heating it for several seconds at a temperature that is at least 5* C. above the first temperature, and then stretching the film in a second direction perpendicular to the first direction at a temperature at least 5* C. above the first temperature.

United States Patent t 13,63l,899

[72] lnventor Merle L. Erickson 7 2,876,067 3/1959 Nagel et al...ital/Shrink Film DlG. UX Saint Anthony, Minn. 2,928,132 3/1960 Richards264/289 UX [21] Appl. No. 878,546 3,312,250 4/1967 Sirignano et al.l38/l 71 :iled t d 530v- OTHER REFERENCES e Lowrey, R. D., ShrinkPackagin Grows, in Modern [73] Assignee glfnnezota Mining andManufacturing Packaging Vol. 40, No. 4A p. 268 274? Dec. 1966.

shpfuljmnn. Sweeting, J., The Science and Technology of PolymerConfinumomimpm of pp s". No. Films, John Wiley & Sons, 1968, p. 460-464, 523, 537- 538 513,428, Dec. 13, 1965, now abandoned. PrimaryExaminer-Edward .l. Earls This application Nov. 20, 1969, Ser. No.-Attorneyl(inney, Alexander, Sell. Steldt & Delahunt 878,546

ABSTRACT: Heat-shrinkable films and heat-shrinkable tub- [54]HEAT-SHRINKABLE FILM AND TUBING ing of linear polymeric esters thatshrink at least 2% times as 7 Claims, No Drawings much along one of thelongitudinal and transverse axes of the [52] Cl D i l 138/171 films andtubing as they do along the other. The films and tub- 1 61 /4 ing shrinkbetween 2 and 20 percent in the direction of least 511 int. Cl Bsb 53/00shmkage and least 20 Percent in dimm" 0f 501 Field of Search 138/118,Shrinkage- A film inventim' is by stretching it in a first direction ata temperature that is above the second 161/46 264/289 order transitiontemperature for the polymer of the film, then thermally conditioning thefilm while it is in the stretched con- [5 References Ch dition byheating it for several seconds at a temperature that is UNITED STATESPATENTS at least 5 C. above the first temperature, and then stretchingthe film in a second direction perpendicular to the first t t t] t5C thf tt 2,784,456 3/1957 Grabenstein 133/ ata empem urea eas above 6 "s cmHEAT-SHRINKABLE FILM AND TUBING REFERENCE TO RELATED APPLICATION Thisapplication is a continuation-in-part of the application Ser. No.513,428, filed Dec. 13, 1965, now abandoned.

BACKGROUND OF THE INVENTION Heat-shrinkable polyethylene terephthalatefilm has recognized properties that have led others to incorporate it inthin wall heabshrinkable tubing useful for such functions as electricalinsulation, physical protection, and closure seals. In one commercialform a narrow strip of heat-shrinkable polyethylene terephthalate thathas balanced length and width shrinkage has been spirally wrapped andadhesively bonded into tubing that exhibits a substantial longitudinalas well as radial shrinkage. This characteristic of balanced shrinkage,however, plus a tendency for the tubing to distort on shrinkage has madeit difficult to provide a smooth, tight covering on objects. Further,the large longitudinal shrinkage requires use of tubing longer than thearticle to be covered and makes it difficult to choose the right lengthof tubing.

Others have suggested cylindrically folding about the longitudinal axisa strip of heat-shrinkable polyethylene terephthalate film that has beenoriented in the transverse direction only. When exposed to heat thisfilm shrinks substantially more in the direction of orientation, whichis the radial direction when the film is formed as tubing; seeGrabenstein, U.S. Pat. No. 2,784,456. This tubing has the advantage oflow longitudinal shrinkage but also has the disadvantages of low tensilestrength in the longitudinal direction and a marked tendency towardfibrillation, and so far as is known, has never been marketed.

What has been needed to provide high-strength heatshrinkable tubing withdesirable shrinkage characteristics is a film that has not previouslybeen available, namely, a biaxially oriented film that has highshrinkage in one direction (at least about percent) and low shrinkage inthe other direction (at least less than 20 percent) and shrinks at least2% times more in the first direction than in the second direction.Tubing made by radially folding such a film about its longitudinal axiswould have the characteristics needed for shrinking the tubing around,for example, a metal-can-enclosed electrical capacitor, a bundle ofcables, or a cable splice. Generally some minimum longitudinalshrinkage, as about 2 or preferably 5 percent, is needed to obtain thebest results; such shrinkage is needed, for example, for the ends of thetubing to shrink down and cover the transverse ends of a can-enclosedcapacitor.

No prior biaxially oriented polyethylene terephthalate film has everexhibited such properties. A typical previous heatshrinkable biaxiallyoriented polyethylene terephthalate film is described in Nagel, U.S.Pat. No. 2,876,067. The film taught in that patent, which is anonheat-set polyethylene terephthalate film prepared by stretching thefilm 3 times in each direction, shrinks when immersed in boiling waterin the neighborhood of percent in the first direction of stretch andabout 50 percent in the second direction of stretch. Richards, U.S. Pat.No. 2,928,132, suggested substantially reducing the amount of the firstdirection stretch to make polyethylene terephthalate film havingbalanced shrinkage. But, insofar as is known, no one has previouslysuggested film that has the kind of highly imbalanced controlledshrinkage described above, and no one has previously suggested how tomake such a film.

SUMMARY OF THE INVENTION The present invention provides flexible, tough,highstrength biaxially oriented film, and tubing made from that film,that is biaxially differentially heat-shrinkable within a wide range ofproportions. More specifically, the heatshrinkable film of thisinvention comprises a flexible, tough, biaxially oriented film oflinear, orientable, crystallizable, heat-settable polymeric ester, whichfilm is nonfibrillating and has a tensile strength in the mutuallyperpendicular directions of at least 10,000 pounds/square inch, and atabout C. (300 F.) has a heat shrinkage in the first of the mutuallyperpendicular directions of between about 2 and 20 percent and at least20 percent in the second direction and shrinks at least about 2% times,and preferably at least 3 times, as much in the second direction asinthe first direction. (By heat-settable polymer is meant a crystallizablepolymer, which when in an oriented film or fiber form, can be heated toa temperature above the orientation temperature but below the polymerscrystalline melting point to reduce shrinkage at temperatures below thetemperature of heating without substantially reducing the tensilestrength of the film.)

This novel film is prepared by accompanying the different axialorientations of the film with different thermal conditioning. The filmis stretched in a first direction, then partially heat-set or thermallyrelaxed while it is maintained in a stretched condition, then preferablycooled, and then stretched again in the second direction. The film maythen be again partially heat-set or thermally relaxed to further modifythe amount of shrinkage in the final film.

As a result of the thermal conditioning of the film after the firstaxial stretch, the biaxially oriented film develops a large differencein the axial forces of recovery that are exerted when the film isheated. It is theorized that heating the film after the first stretchingoperation while it is in an extended condition produces changes in thecrystallinity of the film that in turn reduce the amount of shrinkagethat the film exhibits in the first direction when heated. By the sametoken it is surmised that as a consequence of the increasedcrystallinity of the film, more work is required to stretch the film inthe second direction. The increased work is in turn realized as anincreased force of recovery in that direction that leads to a furtherdifference in the mutually perpendicular amounts of shrinkage.

Heat-shrinkable tubing is made from the novel film of this invention bycylindrically folding strips of the film about their longitudinal axisand then bonding the overlapped edges of the film. Preferably theoverlaid edges of the film are welded by the mechanical impacts of ahammer oscillating at an ultrasonic frequency. Welds prepared in thismanner have been found to avoid the puckering and distortion that occurwith heat-sealing methods used in the past and have been found to givewelds of much higher strength, in fact, of a strength nearly equal tothat of the film itself. Tubing is prepared from film having theshrinkage characteristics described above, and preferably from film thatshrinks between about 40 and 50 percent in the width direction andbetween about 5 and 20 percent in the length direction. Tubing of theinvention may also be made by extruding a tube and subjecting it to thestretching and thermal-conditioning operations described above.

DETAILED DESCRIPTION Polyethylene terephthalate is especiallywell-adapted to the processes of this invention and finds wide usefulapplications after preparation. However, biaxially differentiallyheatshrinkable film and tubing are formed by the processes of thisinvention from other valuable linear, orientable, crystallizableheat-settable polymeric esters, which are defined for the purposes ofthis specification, as those polymeric esters in which dibasic acidssuch as terephthalic, isophthalic, sebacic, bibenzoic, and 2,6-naphthoicacid are a principal acid component.

The biaxially differentially heat-shrinkable film is typically preparedon conventional continuous line apparatus in which the film is firstextruded on a rotating drum and stretched in the machine direction witha conventional length orienter that includes initial, slowly drivenrolls; a bank of idler rolls; and terminal, fast-driven rolls, and inthe transverse direction in a conventional tenter in which the film isheld by clamps carried on chains travelling on diverging tracks. Thefilm may be passed through either the length orienter or the tenterfirst depending upon whether film that is preferentially shrinkable inthe machine direction or the transverse direction is desired. In betweentravel through the first and second stretching stations the film travelsfor several seconds through an oven in which hot air is forced on thefilm while it is held in its extended condition. Preferably the film iscooled after leaving the intermediate oven, as by passage underhigh-velocity air-impingement nozzles. As noted above, the film may alsobe passed through an oven after the second stretching operation, whereit is partially heat-set or thermally relaxed again to further controlthe amount of shrinkage in both directions of the film.

During the first stretching operation the film is generally heated to atemperature above, but not more than 25 C. (45 F.) above, the secondorder transition temperature. (Unless otherwise specified, thetemperatures used in discussing the film processing are ambienttemperatures, that is, the temperature of the atmosphere surrounding thefilm or the temrolls of the length orienter should not be heated to atemperature more than a few degrees higher than 85 C. (185 F Theduration and temperature of the intermediate partial heat-settingoperation are determined by the degree of imbalanced shrinkage desiredand also by the difficulty in uniformly stretching the film after thisoperation. In general, the film is heated in an oven for several seconds(4-5 seconds being a typical period of time) at a temperature at leastabout 5 C. F.) above, and preferably at least 10 C. F.) above, and lessthan C. (45 F.) above the temperature at which the film was stretched inthe first direction. Preferably, the film is then cooled by blowing overit air at high velocity that is at a temperature below the firstorientation temperature. The second stretching step should be performedat a temperature at least 5 C. (10 F.) above the temperature of thefirst stretching operation to give a good film with imbalancedshrinkage. However, the film should not be heated to it is preferablystretched between about 2.8 and 3.3 times in the transverse directionand between about 2.8 and 3.2 times in the machine direction.

The invention is further illustrated in the following examples.

EXAMPLES l-6 In these examples polyethylene terephthalate film wasextruded in a l6-mil (0.4 millimeter) thickness onto a rotating drum andthen length oriented in a length orienter, heated in an intermediateoven while in the stretched condition, cooled under air-impingementnozzles, and width oriented in a renter. The temperature conditions andstretch ratios are given in the following table together with theshrinkages in air at 150 C. for each of the films and the shrinkages inboiling water for some of the films. Also given are the filmtemperatures at the beginning and end of the divergent (stretching)sections of the tenter, measured with a Model lT-3X/3" BarnesEngineering Co. infrared thermometer. Stretch ratios for the transversestretch were determined by measuring the distortion of a 10- inch-squaregrid marked on the film after ishad passed through the length orienter.

Test results for comparative examples A and B are also included in thetable to show the differences between the shrinkage characteristics of afilm of this invention and the previously discussed prior artheat-shrinkable films, that is, the nonheat-set films of U.S. Pat. Nos.2,876,067 and- 2,928,132. These films were made on equipment asdescribed above and with the processing conditions set out in the table;polyethylene terephthalate was extruded onto the rotating drum in a9.7-mil thickness for the example A film and in an 8- mil thickness forthe example B film. As seen from the table, films of the invention havea high imbalance of shrinkage in boiling water as well as at 150 C.adapting the films to uses in which shrinkage is obtained by immersionin boiling water; generally, films of the invention shrink in boilingwater at least 2% times more in one direction than in the other mutuallypera temperature above about 130 C. (270 F.) and preferably-"particulardirection.

Film temperatures Ambient temperatures C.) during shrinkages in Shrinkes transverse boiling water at 150 Length- Width- Interstretch- 0.(percent) (percent) stretch stretch Lengthmediate Cooling ratio ratioorienter oven blower Tenter Start End Length Width Length Width Example:

1 2. 7 2. 3 85 108 91 98 110 10 46 2. 7 2. 4 91 87 90 101 107 15 36 1544 2. 8 2. 5 85 99 82 94 98 12 32 13 41 2. 8 2. 1 85 99 78 94 94 115 942 3. 0 2. 3 85 99 84 94 99 116 11 46 6 3. 0 2. 3 85 98 81 95 95 117 1338 15 46 Comparative Examples:

A 3.0 3.0 84 Ofi Off 87 28 49 27 44 B 2. 4 3. 8 87 Off 011 85 41 41 4338 not above about 120 C. (250 F.), during the second EXAMPLE 7stl'etchmg operation to assure that the film W111 exhibit the In thisexample heat-shrinkable tubing was prepared from 2- desrredheat-shrinkage charactenstrcs at practical shnnkrng mi] ()5-mllhmeter)blaxlally oriented, i i ll dif.

temperatures. For the best width-shrinkable polyethylene terephthalatefilm, the film is preferably not heated during the second stretchingoperation more than about 20 C. (35 F.) above the temperature of thefirst stretching operation.

Though it is difficult to prepare satisfactory biaxially orientedbiaxially differentially heat-shrinkable polyethylene terephthalate filmby stretching the film 2 times or less in the first direction,satisfactory results are obtained when the film is stretched greaterthan 2 times but not more than about 3% times in the first direction andbetween about 2 times and 4 times in the second direction. To obtain thegreatest imbalance of shrinkage for width-shrinkable polyethyleneterephthalate film, the film is stretched between about 2.6 and 3.2times in the machine direction, and then stretched in the transversedirection some amount less, preferably 10 percent less, than the amountof stretch in the first direction. If the film is first stretched in thetransverse direction, on the other hand,

ferentially heat-shrinkable polyethylene terephthalate film that hadlength and width shrinkages at C. of 15 and 45 percent respectively. Astrip of this film 4.6 centimeters wide was cylindrically folded aboutits longitudinal axis and the circumferentially overlaid edges passedalong a cylindrical rod having a diameter of 1.32 centimeters andunderneath the hammer of a 400-watt Model 400, ultrasonic sealer made byUltrasonic Seal, Inc. The hammer was oscillated at 45-percent power at afrequency of 20,000 cycles/second through a deflection of 0.3millimeters, and a 0.0l8-millimeter amplitude at a force of 1.8kilograms. Folded film was passed underneath the sealer at a rate of 50centimeters/minute. When heated at 150 C. this tubing exhibitedunrestrained radial and length shrinkages of 45 and 15 percentrespectively.

Film can also be prepared by welding radially overlaid edges ofcylindrically folded film. The conditions of the ultrasonic sealeroperation can be varied substantially and can be used with a variety ofbiaxially differentially heat-shrinkable films. The rate of feeding thefilm depends on the power output of the sealer.

What is claimed is:

1. Flexible, tough high-strength biaxially differentially heatshrinkablefilm comprising biaxially oriented film of a linear orientablecrystallizable heat-settable polymeric ester that is nonfibrillating andhas a tensile strength of at least 10,000 pounds/square inch in themutually perpendicular directions and at 150 C. is heat-shrinkablebetween 2 and 20 percent in a first of the mutually perpendiculardirections and at least about 20 percent in the second direction andheat shrinks in the second direction at least 2% times the amount itshrinks in the first direction.

2. Film of claim 1 in which the polymeric ester is polyethyleneterephthalate.

3. Film of claim 1 that shrinks at 150 C. at least 3 times as much inthe transverse direction as it shrinks in the longitudinal direction.

4. Flexible, tough high-strength biaxially differentially heatshrinkabletubing comprising a thin wall cylindrically formed film of alinear-orientable crystallizable heat-settable polymeric ester that isnonfibrillating and has a tensile strength of at least 10,000pounds/square inch in the mutually perpendicular directions, and at C.is heat shrinkable between about 2 and 20 percent in the longitudinaldirection and at least about 20 percent in the radial direction, andheat shrinks in the radial direction at least 2% times the amount itshrinks in the longitudinal direction.

5. Tubing of claim 4 in which the linear orientable crystallizableheat-settable polymeric ester is polyethylene terephthalate.

6. Tubing of claim 4 that shrinks at 150 C at least 3 times as much inthe radial direction as it shrinks in the longitudinal direction.

7. Tubing of claim 4 comprising a cylindrically folded film having itsedges circumferentially overlaid and ultrasonically welded together.

2. Film of claim 1 in which the polymeric ester is polyethyleneterephthalate.
 3. Film of claim 1 that shrinks at 150* C. at least 3times as much in the transverse direction as it shrinks in thelongitudinal direction.
 4. Flexible, tough high-strength biaxiallydifferentially heat-shrinkable tubing comprising a thinwallcylindrically formed film of a linear-orientable crystallizableheat-settable polymeric ester that is nonfibrillating and has a tensilestrength of at least 10,000 pounds/square inch in the mutuallyperpendicular directions, and at 150* C. is heat shrinkable betweenabout 2 and 20 percent in the longitudinal direction and at least about20 percent in the radial direction, and heat shrinks in the radialdirection at least 2 1/2 times the amount it shrinks in the longitudinaldirection.
 5. Tubing of claim 4 in which the linear orientablecrystallizable heat-settable polymeric ester is polyethyleneterephthalate.
 6. Tubing of claim 4 that shrinks at 150* C. at least 3times as much in the radial direction as it shrinks in the longitudinaldirection.
 7. Tubing of claim 4 comprising a cylindrically folded filmhaving its edges circumferentially overlaid and ultrasonically weldedtogether.